Filter membranes are routinely used in the analysis and preparation of biological samples. The surface of the substrate used in the manufacture of such membranes is purposefully selected to be antifouling, i.e. resistant to non-specific binding of components of the biological sample, and chemically inert. Microbeads and microspheres are used for the isolation and separation of biomolecules from complex mixtures. Reactive molecules are adsorbed or coupled to the surface of the beads or spheres. When the beads or spheres are superparamagnetic biomagnetic separation techniques may be employed. Membranes and microbeads or microspheres can be manufactured from various natural and synthetic materials including glass, metal, e.g. gold, and polymers, e.g. polycarbonate, polyethylene and polystyrene. Polystyrene is commonly used in biological applications as proteins readily adsorb onto its surface. Glass has limited use because of the limited ability to functionalise its surface. It is an object of the present invention to provide a biocompatible method of localising functional moieties to the inert surface of a substrate or at least to provide the public with a useful choice.